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| Life Could Be a DReaM |
| 摘自: linuxinsider.com 被阅读次数: 717 |
由 yangyi 于 2007-04-06 13:08:12 提供 |
Digital Radio Mondiale is a relatively new digital shortwave broadcast format which can be decoded using open source software developed in Germany. With the free WinDRM software, the format is made available to amateur broadcasters. All you need to explore amateur DRM is a radio, a computer and a sound card interface. Rackspace: Managed Hosting backed by Fanatical Support™. Learn more. If you are old enough to vividly recall the Chords hit suggested in the title, I should tell you straight away that this article has nothing to do with things that go "Sh-boom" in the night. The snippet of lyric just seems to fit this article in a way that I'm helpless to adequately explain. (If you have no clue who the Chords were, click here). The "dream" I'm alluding to is an ingenious piece of open source Latest News about open source software developed by Volker Fisher and Alexander Kurpiers at the Darmstadt University of Technology in Germany. It was designed to decode Digital Radio Mondiale (DRM), a relatively new digital shortwave broadcast format. If you've tuned outside the amateur HF bands you've likely heard DRM on the air. It is a wide signal that sounds like the bellow of an enraged chain saw. A commercial DRM signal is capable of carrying high quality audio along with text and occasional images, which is a pretty cool trick for a long-haul circuit on a shortwave frequency. DRM works its magic through -- brace yourself -- Coded Orthogonal Frequency Division Multiplexing (COFDM) with Quadrature Amplitude Modulation (QAM). COFDM uses a number of parallel subcarriers to carry all the information, which makes it a reasonably robust mode for HF use. Redesigned for Amateur Radio Not long after Dream debuted, Francesco Lanza, HB9TLK, began adapting it for amateur radio. He redesigned Dream to support DRM transmission and reception within a 2.5-kHz SSB transceiver bandwidth. That meant sacrificing some audio quality, along with the ability to simultaneously send images and other large chunks of information. Despite the difficulties, Francesco's ingenuity prevailed and WinDRM was born. As the name implies, WinDRM is a Windows application. It is designed to use your computer sound card (or on-board sound chipset) to send and receive amateur DRM on HF, or wherever you choose to use it. Best of all, WinDRM is utterly free of charge. All you need to explore amateur DRM is a radio, a computer (preferably with a 1 GHz processor or faster) and one of the sound card interfaces you often see advertised in QST (West Mountain Radio, MFJ, TigerTronics, MixW RigExpert, microHAM and so on). If you are already active on the digital modes with your sound card, you can give WinDRM a spin right away. Just download the software and go. There is a WinDRM bonus, too. Other digital data/image systems such as Digitrx and HamPAL are 100 percent compatible with WinDRM, so you can use WinDRM to participate in these exchanges as well. Listening With WinDRM Start by downloading and installing the WinDRM software. Most WinDRM enthusiasts are using the MELP codec, so I'd suggest downloading the latest MELP version of WinDRM along with a copy of the MELP codec and the TUNE audio WAV file (under "WinDRM Utilities"). All of these download links are on the same Web page. If you intend to swap digital images, you'll also need to download and install the free Irfanview software. Irfanview is a nice little application that allows you to view and edit a huge variety of digital images. I won't go into detail about how to set up the WinDRM software because Jason Buchanan, N1SU, has already done the work for me. He has an outstanding WinDRM tutorial on the Web. Everything you'll need to know is there. That said, you'll need to make sure that you've made the proper connections between your computer and your radio. If your objective is to simply receive amateur DRM, all you need is a cable between the audio output of your radio and the LINE INPUT of your sound card. You may need to get into your sound card control software and boost the LINE INPUT gain. Using the Windows audio mixer, which you can access within WinDRM, bring up the Recording Control panel and make sure LINE is selected and that the "slider" control is up. If the WinDRM waterfall display is too bright, reduce the gain control. Finding Everyone Else WinDRM users tend to hang out around 14.236 MHz USB Latest News about USB, although you'll find them on other bands as well. When this article went to press, the WinDRM portion of the Weekend Digital Voice Net was meeting on Saturdays and Sundays at 14.236 MHz from 2000 to 2100 UTC. This might be a good opportunity to test your receive capability. Listen for the jet turbine noise and then tune your receiver until you see three vertical lines in the waterfall display. The lines must be centered beneath the red markers at the top of the waterfall window. You'll know you are in the sweet spot when you see the "state" indicators along the left side of the window lighting up in sequence from top to bottom. (Actually, they go black, but let's not quibble.) When the MSC (Main Status Channel) indicator blinks on at the bottom, you should be hearing audio through your PC speakers or headphones. If you can't get all the indicators to light, (A) the signal isn't tuned correctly, (B) you don't have enough audio going to the sound card or the software or (C) the received signal is too weak. When you finally achieve success, the result is an astonishing thing to hear. The voice sounds like it is coming to you through a local FM repeater. Look in the lower left corner of the WinDRM window and you will see the call sign of the transmitting station. If the station is sending text, a separate window will pop open to display the text if you've enabled that function in WinDRM. If you enjoy image communication Get the Facts on BlackBerry Business Solutions, you'll still find traditional analog SSTV (Slow Scan TV) around 14.230 MHz, but move up to 14.233 MHz and you're likely to encounter Digital SSTV (or D-SSTV). As with SSTV, the digital images are normally exchanged during analog SSB conversations where you'll hear someone say, "Hey, Charlie, let me send you this picture," followed by the buzz of a digital transmission. When you hear the buzz, line up the three vertical lines in WinDRM just as you did with the digital voice transmission. If you capture enough data to display a picture, Irfanview will open automatically and show the image on your screen. The quality can be pretty amazing! Ready to Transmit? Transmitting with WinDRM is not as straightforward as listening, especially if you have only one sound card. The reason is easy to understand when you think about it. The audio output from your sound card must be applied to your headset or PC speakers for receiving, but the same output must also feed your transceiver for transmitting. If you are stuck with one sound card, you are going to have to switch the audio output in some fashion. I use the tried-and-true "Armstrong" method. When it is time for me to transmit, I unplug the audio cable from my headset and substitute the cable that snakes to my sound card interface. With the correct cables in place, I click my mouse cursor on the TX button and WinDRM places my radio in the transmit mode. With the microphone portion of my headset plugged into the sound card MIC IN jack, I'm good to go. If you think this arrangement sounds crude, you're right. The easier, more elegant approach is to use two separate sound cards. No desperate yanking and plugging of cables here. Just click the TX button and talk. The second sound card doesn't need to be expensive. In fact, it doesn't even need to be installed inside your computer. Take a look at the tiny US$10 HE-280B USB sound card available at Geeks.com as a prime example. Dreaming on the Air This is new technology on the ham bands and the operating practices are very much in flux. I hear some digital voice aficionados who operate amateur DRM exclusively; they never switch to analog during the conversation. Others begin their CQs in analog SSB like this: "WB8IMY calling CQ in digital voice using the MELP codec." After that short analog preamble, they jump to DRM and call a long digital voice CQ. It is important to point out that DRM isn't the only game in town. If you're a regular QST reader, you've seen the advertisements and reviews of the AOR "voice modems." This is a different approach to digital voice where the software exists as firmware in the AOR box. You plug your microphone into the modem and plug the modem into your transceiver. The AOR devices use a different digital coding scheme that is incompatible with WinDRM, so you can't use WinDRM to talk to a station using an AOR voice modem, or vice versa. Which is better, WmDRM or AOR? The jury is still deliberating that question. Some believe that the AOR voice modems have an edge over WinDRM in less than ideal conditions. On the other hand, WinDRM is far less expensive and experiments are continuing with new codecs that promise to yield better performance in poor signal environments. I've tried both AOR and WinDRM and in my experience they are nearly equivalent. With either one you need good signal conditions at both ends to minimize annoying dropouts. Neither of these digital voice modes tolerate fading or interference very well. I don't see digital voice becoming dominant on the HF phone bands any time soon. The weak-signal advantages of AM and SSB are going to be pretty tough to beat. Still, when signal conditions permit, MnDRM is a heck of a lot of fun. Who ever thought we'd hear noise-free FM-quality voice on HF? In addition, WinDRM is capable of some other neat tricks such as an error-free ARQ data mode for file transfers. Read the NISU tutorials and you'll discover that WinDRM is a highly versatile program To invoke the Chords one final time, "Life can be a DReaM, sweetheart." 原文链接: http://linuxinsider.com/story/6gsTq2hmrsRIWn/Life-Could-Be-a-DReaM.xhtml# |
